1. Field of the Invention
The invention relates to digitizing apparatus and methods, and more specifically to digitizing apparatus and methods for improving accuracy of interpolation between spaced grid conductors in digitizers and compensating for errors due to tilting of a pen and variations in pen height.
2. Description of the Prior Art
Various high resolution apparatus for translating a position of a movable instrument, such as a cursor or a pen, into digital signals for transmission to a utilization device are well known in the art. Such digitizing devices generally include a rigid support structure suitable for supporting a work sheet such as a graph, chart, or piece of industrial artwork to be digitized or a blank sheet upon which a pen can make marks, the location of which marks are to be digitized. A grid or matrix of conductors ordinarily is embedded immediately beneath the support surface. If the movable instrument is a pen, it is positioned over and pressed against the work sheet and is moved to trace various patterns on the work sheet, points of which patterns are to be digitized. If the movable instrument is a cursor, it is aligned with various pre-existing points or marks on the work sheet, which points or marks are to be digitized. The state of the art for digitizers is indicated by U.S. Pat. Nos. 3,767,858, 3,851,097, 3,983,322, 3,886,311 and 4,022,971, all assigned to the present assignee. The state of the art is further indicated by co-pending patent application Ser. No. 921,514 by George A. Fencl, filed on July 3, 1978, and assigned to the present assignee, now U.S. Pat. No. 4,185,165. Digitizers described in the foregoing prior art have relatively high accuracy or resolution. However, the cost of such digitizers increases rapidly as the resolution increases. There are several sources of inaccuracy in the above digitizers, one of the most important being the difficulty in accurately interpolating or determining the position of the pen or cursor when it is located at a point between two adjacent grid conductors. Non-linearity in the characteristic waveform produced by the output of the filter in the above mentioned co-pending Fencl application significantly reduces the accuracy of interpolation obtained by incrementing a pen position counter at a constant rate during the process of scanning the grid conductors in a particular direction. An analogous problem exists in digitizers wherein signals are transmitted from the scanned grid conductors rather than received by them.
Accordingly, it is an object of the invention to provide a low cost, high accuracy digitizer having improved accuracy of interpolation of pen or cursor position at points located between grid conductors.
Another problem with digitizers constructed in accordance with the known prior art is that it is often necessary to decrease the spacing between grid conductors to achieve high accuracy. This greatly increases expense and also increases time required for performing digitizing operations, since a larger number of conductors must be scanned. Another problem of prior digitizers is high power dissipation in the pen or cursor coils.
It is therefore another object of the invention to provide an improved high accuracy digitizer having reduced power dissipation.
Another source of inaccuracy in prior digitizers having pens results from tilting of the pens, causing assymmetrical interaction of magnetic field lines and grid conductors and consequent loss of accuracy of the scanning procedures used.
Accordingly, it is a further object of the invention to provide a digitizing apparatus and method for economically, quickly, and accurately compensating for pen tilt to provide high accuracy digitizing.